Predictive Value of Attenuation Coefficients Measured as Hounsfield Units on Noncontrast Computed Tomography During Flexible Ureteroscopy with Holmium Laser Lithotripsy: A Single-Center Experience

Abstract

To assess the utility of attenuation coefficients as predictors of surgical outcome after a single flexible ureteroscopy (URS) with holmium laser lithotripsy. Many reports indicate that the efficacy of extracorporeal shockwave lithotripsy (SWL) can be predicted by the target's radiofrequency attenuation, measured as Hounsfield units (HUs) on noncontrast CT (NCCT). Studies of flexible URS, however, have not assessed the predictive value of attenuation coefficients on NCCT. Patients with renal stones who were treated by flexible URS with holmium laser lithotripsy between December 2009 and October 2011 at a single institute were retrospectively evaluated. Stone-free (SF) status was determined by kidneys-ureters-bladder (KUB) radiography at postoperative month 3. Correlations of possible predictors with SF status were analyzed using a logistic regression model. The comparison between groups with low and high HUs was examined using the Mann-Whitney U test. There were 219 eligible procedures. According to the logistic regression model, the maximum attenuation coefficient (P=0.105) and average attenuation coefficient (P=0.175) did not significantly, independently predict SF status. Fragmentation efficiency was significantly different between cases with low and high attenuation coefficients (P=0.001). In groups with less than 20.0-mm diameter stones, overall operative time (P<0.001 and P=0.001) and the time from starting fragmentation (P<0.001 and P=0.002) were significantly high in both attenuation groups. In groups with stones greater than 20.0 mm diameter, the two definitions of operative time revealed no differences between the low and high attenuation groups. The retrospective study design was the major limitation of this study. We found that both the maximum and average attenuation coefficients on NCCT are significantly related to the fragmentation efficiency. In addition, this study showed that, in patient groups with stone a burden <20.0 mm in diameter, both the maximum and average attenuation coefficients were significantly predictive of operative time.